1. Field of the Invention
The invention is generally directed to apparatus which are utilized for the purpose of altering or shaping the surface configuration of land areas. More particularly, the invention relates to a vehicular land plane apparatus for grading and leveling land areas, particularly agricultural fields.
2. Description of the Prior Art
Land planes are well known in the prior art and typically comprise a wheeled frame carrying at least one grader blade that is adapted to contact and smooth the land surface. These apparatus are usually pulled by a separate towing vehicle such as a tractor or the like. Because land planes are primarily utilized for grading and shaping large areas of land, they are necessarily very large and specialized apparatus which are characterized by many requirements not normally encountered by similar apparatus or vehicles of smaller physical stature.
A land plane is generally composed of a large frame structure which is supported on steerable wheels so that the vehicle may physically negotiate about the entire area to be leveled. The frame structure, because of its great size and weight, inevitably undergoes warping or twisting due to the severe mechanical stresses realized during use. In attempting to overcome the undersirable warping of the frame structure, the prior art has suggested constructing the frame in various heavy and rigid truss designs. Such designs are supposed to keep warping and twisting of the frame to a minimum so that the grader blades attached thereto can be individually raised and lowered with respect to the ground surface in order to provide the proper blade alignment and grading depth.
The great length and substantial width of typical land levelers require specialized steering systems to safely accommodate effective steering of these vehicles during actual use and highway travel. This has normally been accomplished by providing a pair of steerable front wheels, the steering movements of which are usually directly coordinated with a pair of steerable rear wheels through rigid cables or mechanical linkages. In this way, turning of the front wheels in one direction will automatically cause the rear wheels to turn the same degree, though in an opposite direction, so that the distant rear end of the apparatus may negotiate a desired change in direction.
Another requirement of these specialized apparatus is the provision of some means for either raising the grader blades suspended beneath the frame or raising the entire frame with the blades attached thereto to a sufficient height to permit road travel or clearance of ground surface obstructions. The prior art has proposed to satisfy this requirement in several ways. For example, it is known to mount the grader blades to a subframe which is independently hinged to the lower portion of the primary frame so that the subframe may be lowered and raised with respect to the primary frame, the latter being supported at a comparatively high distance off of the ground surface. It is also known to construct the frame in pivotally connected sections, including a center section and two end sections, whereby pivoting of the end sections will cause the center section to be raised and lowered, along with its attached grader blades, while maintaining itself parallel with respect to the ground surface.
Exemplary of typical prior art devices which provide some indication of the state of the art to which the present invention pertains are the Kenney U.S. Patent No. 2,994,143, Saul U.S. Pat. No. 3,081,567, Constantin U.S. Pat. No. 3,172,218, Purdy U.S. Pat. No. 3,348,323 and Estes U.S. Pat. No. 3,800,884.